ENGINE OVERHEAT

I have replaced a radiator (after market), hoses, radiator cap and the clutch fan assy (after market) were fitted in the hope of eliminating engine overheat and cooling boiling. I have replaced the head gasket and piston rings (no liner change). However, the overheating remains and the smoke is thick white especialy at idle or when going downhill on low gear. It also backfires when going down hill at low gear.

The other problem also is that engine revs remain very high after running the car for some distance (note that at starting, the engine revs are okay.)

I have tried all my best to get the car in good condition and in doing so, I spend spend more than US$5000. Please help.